The present invention relates to an improved method for separating a spent or partially spent mixed-resin bed into cation-exchange resin and anion-exchange resin portions for separate regeneration with an acid and a base, respectively. The present invention finds particular utility in a "condensate polishing system" used in conjunction with a steam generating unit, e.g., a steam turbine power system, for the purpose of removing trace corrosion and erosion products from condensate water. The "condensate polishing system" will consist of one or more service units and at least two regeneration units. When the ion exchange resin in the service units becomes exhausted in terms of its capacity to remove dissolved solids or suspended solids, i.e., anions and cations, as indicated by detection of a lower than predetermined effluent quality, or when the pressure drop in a service vessel exceeds a certain predetermined value, the service cycle is considered terminated. At this time, it becomes necessary to switch the condensate feed to another service vessel and to chemically treat the exhausted mixed resin for regeneration. The exhausted ion exchange resin is conventionally hydro-pneumatically transferred to the regeneration units.
It is common practice in the art to separate the anion-exchange resin from the cation-exchange resin for separate chemical regeneration by treatment with a base and an acid, respectively. As is well known to those skilled in the art, such separation is conventionally accomplished by passing water upwardly through the spent, mixed-resin mass to hydraulically stratify the resin, forming a floating bed of the less dense anion-exchange resin in the upper portion of the separation vessel and another floating bed of the more dense cation-exchange resin in the lower part of the vessel. These stratified and separated resin beds are then separated for chemical regeneration.
However, when the resins are hydraulically classified in the conventional manner, inevitably some cation resin is carried over with the anion resin in the form of either whole bead or fines and will thereby come into contact with the base, e.g., NaOH, during the anion regeneration. Stoke's law holds that the settling velocity of a particle is a function of its specific gravity and also its diameter. Since resin particles regurlarly have certain range of sizes, some small cation particles may settle at the same level as some larger anion particles simply because both have the same settling (rising) velocity. This is the basic reason why backwash separation will not get a clean cut separation. In the anion regeneration process, sodium sites will form on any cation resin fines which are present, which when returned to a service vessel and placed in a high pH enviornment, will leak through sodium ions. This leakage of sodium ions can cause damage to a steam turbine power system.
Several patents have been issued covering means for dealing with the presence of cation-exchange resin fines in the anion-exchange resin regeneration zone. For example, U.S. Pat. No. 4,039,443 issued to Sentaroo Tahara et al, discloses treatment of the anion-exchange resin with an aqueous solution of hydrazine, after regeneration with caustic soda. U.S. Pat. No. 3,385,787 issued to Crits et al, U.S. Pat. No. 3,501,401 issued to Calmon et al, U.S. Pat. No. 3,583,908, issued to Crits and U.S. Pat. No. 3,709,818 issued to Gustafson et al also disclose a further chemical treatment of the anion resin to obviate the effect of the presence of cation fines. These supplemental chemical treatments are unsatisfactory for the reason that they add an additional recurring cost.
U.S. Pat. No. 3,582,504 issued to Salem et al and U.S. Pat. No. 3,826,761 issued to Short, also address the problem posed by the presence of cation fines in the anion resin and propose a process for the physical separation of those fines which utilizes a liquid having a density intermediate the density iof the anion exchange resin and that of the cation exchange resin. However, the use of a special density liquid introduces another separation problem as well s other process problems and cost.